Organic Mechanisms 1 Concepts The key ideas required to understand this section are: Concept Book page Chemical properties of alkanes 314 Chemical properties of alkenes 318 Bonding in alkenes 320 Bonding in benzene 323 Chemical properties of benzene 324 Mechanisms of organic reactions A reaction mechanism is a step-by-step description of the order in which bonds break and atoms rearrange during a reaction. A mechanism proposed by a chemist must take into account all the known facts about a reaction if a mechanism does not fit in with any new facts discovered it is discarded or refined. Many mecha nisms, however, have been sufficiently tested to become part of the theory of organic chemistry. Why do we study mechanisms in organic chemistry? There are very many reactions in organic chemistry and studying mechanisms helps us to: (a) make sense of (i.e. see patterns in) the reactions; (b) change the experimental conditions under in order to increase the yield of products; and (c) predict what might happen in a reaction. Types of organic reactions There are four types of reaction: (i) Substitution reactions (where one atom, ion or group is substituted for another) e.g.: X + AB AX + B 1
(ii) Addition reactions (where one reagent is added to another without the loss of any other atoms). Alkenes undergo addition reactions, where a reagent adds across the double bond: (iii) Elimination reactions (the reverse of addition; atoms or groups are removed from a molecule creating a multiple bond): (iv) Rearrangements (where an atom or group goes from one position in a molecule to another): Key factors in reaction mechanisms (i) Bond-breaking A covalent bond between two atoms can be broken in three ways: Free radicals are atoms, or groups of atoms, having an odd unpaired electron. (ii) Attack! (or love at first sight?) A reagent that is attracted towards a region of high electron density (an electron pair) is called an electrophile ( electron lover ). Electrophiles are often positively charged; e.g.: H3O+, NO2+ (the nitronium ion). Sulfur trioxide, SO3 is also an elec trophile. A reagent that possesses a pair of electrons that it is anxious to share with a nucleus that is short of electrons is called a nucleophile ( nucleus lover ), e.g.: OH, H2O, Br, NH3. Because these two reagents find what they are looking for in each other, they have a natural attraction: X+ + Y XY electrophile + nucleophile + product Curly arrows are used in equations to show the movement of electrons; e.g., in heterolytic fission: The tail of an arrow shows where an electron pair moves from, the head of the arrow shows where it moves to. If a covalent bond breaks, the tail of the arrow starts at the middle of the bond. A half arrow or fishook is used to show the movement of one electron: 2
Mechanism of the chlorination of methane Methane and chlorine do not react together when they are in darkness, but in sunlight the reaction between them is very vigorous. The chlorination of methane in sunlight is an example of a substitution reaction, and the overall equation is: CH4 + Cl2 CH3Cl + HCl The reaction proceeds by the following steps: In the presence of sunlight (ultraviolet light), the chlorine molecule undergoes homolytic fission and two free radicals are formed: Cl2 2Cl (the initiation step). This step could also be written The chlorine radical then takes a hydrogen atom from methane, hydrogen chloride and a methyl radical are formed: CH4 + Cl HCl + CH3 The methyl radical then takes a chlorine atom from a chlorine molecule and chloromethane and another chlorine radical is produced: CH3 + Cl2 CH3Cl + Cl The chlorine radical formed can now take another hydrogen atom from either another molecule of methane or a molecule of chloromethane. These last three reactions keep the process going because although each step uses up a free radical, it produces one as well (they are propogation reactions). The process is called a chain reaction. The reaction chain ends when two free radicals collide and combine (a termina tion step). This can happen in a number of ways: Cl + Cl Cl2 CH3 + Cl CH3Cl or even: CH3 + CH3 C2H6 Because a mixture of products (C2 H6, CH3Cl, CH2Cl2, CHCl3 and CCl4) can result from these reactions, you can see that this reaction is not really a suitable method to prepare chloromethanes in the laboratory. Addition reactions of alkenes (i) With bromine Ethene and bromine react to form 1,2-dibromoethane: 3
A bromine molecule is non-polar, but as it approaches the ethene molecule it becomes polarized; the electrons in the - bond of ethene repel the electrons in the bromine molecule: The positive end of the bromine molecule attacks the double bond and a bridged bromonium ion is formed. This addition reaction therefore starts by the electrophilic attack of the positive end of the bromine molecule on ethene. The positive bromonium ion is rapidly attacked by the negatively charged bromide ion and 1,2-dibromoethene is formed. (ii) With hydrogen bromide 4
When hydrogen bromide reacts with ethene, bromoethane is formed: Hydrogen bromide is a polar molecule, H δ+ Br δ. The first step in the reaction is electrophilic attack of a proton on the π-bond of ethene and the H Br bond breaks. A positively charged carbocation is formed, which reacts with the bromide ion to form bromoethane. The stability of carbocations Carbocations are organic cations with the positive charge carried on the carbon. If propene reacted with HBr, there would be two possible carbocations: products: giving two possible In fact, much more 2-bromopropane is formed, because carbocation CH3 C + H CH3 is the more stable of the two possibilities; it contains two alkyl groups ( CH3) which tend to push electron density on to a carbon atom joined to them. The more electron density that is pushed on to the carbon with the positive charge, the more stable is the cation. CH3 C + H CH3 CH3 CH2 C + H2 two alkyl groups stabilising one alkyl group stabilising the carbocation the carbocation 5
The addition of a hydrogen halide to a double bond is described by Markownikov s rule: Markovnikov s rule predicts the major product when HX (X = Cl or Br) reacts with an alkene: the major product is the one in which the hydrogen atom attaches itself to the carbon atom carrying the larger number of hydrogen atoms. The nitration of benzene A mixture of concentrated sulphuric acid and nitric acids added to benzene results in the formation of nitrobenzene. This is a substitution reaction the nitro group is substituted for a hydrogen atom in the benzene ring: The reaction between concentrated sulphuric and nitric acids is shown by the following equation: HNO3 + 2H2SO4 NO2 + + H3O + + 2HSO4 The powerful electrophile NO2 + is produced, which attacks the clouds of electron density above and below the benzene ring then bonds to one of the carbon atoms in the ring. The doughnuts of delocalized electrons are disrupted and the intermediate cation formed immediately breaks down into nitrobenzene so that the more stable delocalized system can be re-formed; in the process it gives up a proton to HSO4, producing H2SO4 again. The reaction is classified as an electrophilic substitution reaction. Other electrophilic substitution reactions include halogenation, alkylation and acylation. Halogenation Here a halogen (e.g. chlorine) is substituted for a hydrogen atom on a benzene ring. The reaction takes place in the presence of a catalyst, such as iron (III) chloride, which draws electron density from the halogen molecule towards it. The halogen molecule becomes polarized, its positive end acts as an electrophile and attacks the benzene ring: 6
Alkylation (a Friedel Crafts reaction) A halogenoalkane reacts with benzene in the presence of a catalyst, and one of the hydrogen atoms on the benzene ring is replaced by an alkyl group. For example, chloromethane reacts with benzene in the presence of catalyst aluminium chloride and toluene is formed: The mechanism is similar to that of electrophilic substitution reactions already discussed and involves attack by the positive end of the complex CH3 + Cl -.AlCl3. Aluminium chloride draws electron density from chloromethane and further polarizes the molecule. 11 11. Acylation (another Friedel Crafts reaction) An acid chloride reacts with benzene, in the presence of aluminium chloride, and a ketone is formed (a hydrogen atom on benzene is replaced with a COR group). For example: The attacking electrophile is the polarised complex: δ + δ- H3CCOCl.AlCl3 12 Aromatics and Hűckel s Rule. Aromaticity is a chemical property in which a planar ring of alternating single and double bonds (according to its Kekulé structure), has more stability than would be expected. Benzene is the parent compound of this class, but there are others and lone pairs from atoms other than carbon can be included. The stability comes from all atoms in the ring having an equal share of electrons which are delocalised in an electron cloud. 13 Hűckel s Rule This states that a planar molecule, in which all atoms contribute electrons, that has 4n + 2 delocalized electrons (where n is an whole number) will have aromatic properties. For example benzene has a planar framework (see p323) where each carbon uses three of its valence electrons to bond to two other carbons in the ring and a hydrogen atom: 7
There is one p electron left over on each carbon atom and these delocalise into an electron cloud above and below the ring: This is in accordance with Hűckel s Rule, where there are 6 delocalised electrons: i.e. 4n+2 where n = 1. 14 Is naphthalene aromatic? The Kekulé structure of naphthalene is shown above. Here, we have another system of alternating single and double bonds with 10 electrons (one on each carbon) available for delocalisation. If n= 2, then 4n+2 = 10 so naphthalene is aromatic. 15 When a lone pair is involved eg furan. Carbon can form rings in which an atom of another element is involved. These are called heterocyclic molecules. Furan is an example: It is aromatic and obeys Hűckel s Rule; here oxygen has two lone pairs and one of those pairs delocalises with the single electrons available on each of the four carbons: Ie 4 + 2 = 6 or 4n+2 when n= 1. 8
Revision Questions 1. A compound found in the gasoline fraction of crude oil is decane, C 10 H 22. (i) Write a balanced equation for the complete combustion of decane in oxygen. (ii) What products would you expect to find as a result of the incomplete combustion of decane? 2. But-1-ene was reacted with a solution of bromine, the bromine was in excess. (i) Assuming the dibromoalkane is the sole product of this reaction, write a balanced equation for the reaction. Give the systematic name of the product. (ii) If 15 g of but-1-ene produced 35 g of halogenated product, calculate the percentage yield of the reaction. 3. (i) Write the structural formula of propene. (ii) Write the structural formula and name the product formed by polymerization of this alkene. 4. Explain why ethene reacts with bromine very rapidly, but the reaction of benzene with bromine is slow unless AlBr3 is present. 5. Which of the following molecules are aromatic? 6. W hich of the following heterocyclic molecules are aromatic? 9
Answers Exercise 17A (i) The bridging bromine atom is in the way of the carbon atoms on one side there is steric hindrance by the bridging atom. (ii) trans-1,2-dibromoethene Exercise 17B The possible carbocations are: 2. (i) CH 2 =CHCH 2 CH 3 + Br 2 CH 2 BrCHBrCH 2 CH 3 1,2-dibromobutane. (ii) 1 mol of CH 2 =CHCH 2 CH 3 produces 1 mol of CH 2 BrCHBrCH 2 CH 3 ; 56g of CH 2 =CHCH 2 CH 3 produces 216g CH 2 BrCHBrCH 2 CH 3 ; and 216 15 g 15 g of CH 2 =CHCH 2 CH 3 produces 56 CH 2 BrCHBrCH 2 CH 3 = 58 g theoretical yield of CH 2 BrCHBrCH 2 CH 3 is the more stable because three alkyl (methyl) groups are pushing electron density on to the positive carbon atom. The major product would therefore be: CH CBr CH3 CH3 Exercise 17C The overall reaction is Therefore the percentage yield obtained = actual yield 100 = 35 100 = 60%. theoretical yield 58 3. (i) CH 2 =CHCH 3. (ii) -CH 2 -CHCH 3 -CH 2 -CHCH 3 -CH 2 -CHCH 3 - CH 2 -CHCH 3 -CH 2 -CHCH 3 -, polypropene. 4. Ethene reacts rapidly in an addition reaction: CH 2 =CH 2 + Br 2 CH 2 Br-CH 2 Br The electron density in benzene is delocalized, therefore it does not undergo addition reactions so readily. In the presence of AlBr3 benzene undergoes substitution: The mechanism is as follows: (ii) C, CO in addition to CO2 and H2O. 5. (i) No, there are 8 electrons (one on each carbon) available for delocalisation. This does not conform to Hűckel s Rule (4n+2). The shape of cyclooctatatraene is puckered and non- planar. (ii) Yes, each carbon atom donates 1 electron and there is an extra negative charge making 6 electrons in all. 10
6. ( (iii) Yes each carbon provides 1 electron making 14 this is 4n+2 if n= 3. (iv) Yes, the positive charge indicates an electron has been lost the are therefore 2 electrons, one on each of the carbons that are not charged. This conforms to Hűckel s Rule if n= 0. (i) Yes, pyridine is aromatic. Nitrogen has a lone pair of electrons, but they do not get involved in the delocalised ring of electrons each carbon contributes 1 electron and nitrogen, which has 5 valence electrons also donates I electron making six. The lone pair on nitrogen is available for bonding and this makes pyridine basic.(ii) Yes, sulfur donates 2 electrons (as oxygen does in furan) and there is 1 electron per carbon atom making six in all. (iii) Yes, nitrogen has a lone pair of electrons available, which together with 1 from each carbon atom, makes 6. (iv) No the carbons are bonded to 2 hydrogens and 2 carbons each so cannot contribute any electrons. Rob Lewis and Wynne Evans, 2001, 2006, 2011, Chemistry, Palgrave Macmillan. 11